Method of coating substances



Patented Aug. 11, 1931 UNITED STATES PATENT OFFICE HUGH S. COOPER AN DMENAHEM MERLUB-SOBEL, OF CLEVELAND, OHIO, ASSIGNORS TO KEMETLABORATORIES, INC., A CORPORATION OF NEW'YOBK mnrnon or conrnvesuns'rnncns No Drawing.

, This invention relates to a method of coating substances with metal ormetals of the alkali or alkaline-earth groups, and with the oxids of'such metals. The invention is more particularly applicable to theformation of electron-emitting elements or cathodes (whether of thefilamentary or indirectlyheated type) suitable for use in vacuum tubessuch as audion tubes, X-ray tubes, and the like. Although the inventionis not limited to metallic core, usually in the form of a wire or amethod of making such elements, it will hereafter be described in thatconnection, by way of suitable example.

Cathodes of the dull-emitter or Wehnelt type ordinarily consist of aplatinum or other ribbon, which has been coated with oxid of some metalor metals, such as barium, strontium, or the like. Several methods haveheretofore been employed in the production of such cathodes. One of theolder processes is to dip the wire to be coated into a solution of thenitrate of the desired metal or metals,

and then decompose the nitrate by heating inv an inert atmosphere inorder toobtain the oxid. It is ordinarily necessary to apply aconsiderable number of successive coats of oxid in this manner if auniform electron emission is to be obtained from the finished element.In another method of producing an oxid coating theheated wire is broughtinto contact with solid parailine in which is suspended a considerableamount of barium, or

strontium carbonate, or the like. The carbonate being decomposed, anoxid coating remains. In this case, also, a large number of formelectron emission is desired, which is troublesome and expensive initself, but there is the further difiiculty that considerableApplication filed September 20, 1927. Serial in. 220,831.

uantities of gas are apt to be occluded during t e coating process. Itis extremely diflicult, or impossible, completely to remove thisoccluded gas during the subsequent evacuation of the tube, and this mayaccount, to some extent, for the frequent erratic behavior of cathodesmade in this Way. Moreover, it is diificult to make the coatings ofuniform thickness; and hot-spots and consequent burning out of thefilament frequently occur. With regard to the third process, bariumazide is a highly explosive compound; its preparation is correspondinglydiflicult, and the process is so dangerous as to be highlyunsatisfactory.

Our invention provides a means for applying metallic or oxid coatings tosubstances of any desired character. The method is substantially freefrom the foregoing ditficulties and objections, and results in a moreuniform and adherent coating than has been (so far as we are aware)hitherto obtainable.

According to a preferred method of practising our invention we firstprepare a solution of the desired alkali or alkaline-earth metals in avolatile substantially anhydrousammoniacal liquid. We then apply thismetal solution to the surface of the substance to be coated, and exposethe surface to the air at ordinary temperatures. The solvent quicklyvaporizes, leaving a surface film of the dissolved metal or metals. Thisfilm readily oxidizes on continued exposure to the air, forming a firm,evenly distributed, uniform, and adherent coating of oxid.

A particular example of one method of carrying out our invention is asfollows:

Anhydrous liquid ammonia is placed in a Dewar flask and an excess ofbarium metal is added thereto. A characteristic blue solution quicklyresults, and since the metallic barium is present in'excess, there ma bea second and more concentrated phase, or example in the nature of asuspension, as well. It has been established, as we believe, that thebarium is present in an ionized condition,

since free electrons are liberated in the solution. This solution isthen applied, by dipping, painting, or otherwise, to the surface of afilament wire, for example, which 1t is desired to coat. The filament isexposed to the air so that the ammonia will vaporize.

A surface film of metallic barium then remains. This may be utilizeddirectly for cer- 5 tain purposes, but is readily oxidized by furtherexposure to the air, in which case an oxid skin or coating having thegenerally desired properties is formed. In some cases'we prefer to heatthe filament, as by passing an electric current through it, in order tobring about an even firmer bond between the oxid coating and the metal.The necessity for this heat treatment, as well as the optimumtemperature and duration, depends some- 5 what upon the metal to whichthe coating is applied and the oxid or oxide which compose the coating.Obviously, the exact conditions cannot be specified in advance, but maybe readily determined for any given case.

As an alternative to anhydrous liquid ammonia as a solvent, we havefound that certain alkyl amines, for example methyl amine and ethylamine, may be employed successfully as solvents. We regard such solventsas substantially equivalent, for the purposes of the present invention,to liquid ammonia. The ternrammoniacal liquid as used in thisapplication either in the specification or claims is'to be understood,therefore, as including the several akyl amines as well as liquefiedammonia.

It is not essential to our invention that the metal or metals employedshall be directly dissolved in the solvent, since other methods ofproducing a metal. solution of the characterdescribed are equallyapplicable. For example, a solution in ammonia of a salt of the desiredmetal or metals may be subjected W to electrolysis in order to producethe required metal solution.

, The metal barium is one example only of the several metals which maybe treated according to our invention. Metals of the alkali group, suchas sodium, potassium, caseium, etc., etc.; and metals of thealkalineearth group such as strontium, calcium, etc., which are readilyoxidized, are "soluble in liquid ammonia and form electron emittingoxides, are all suited for use according to our process. The term,oxidizable metal, as used in the specification and claims is to beunderstood as including the alkali metals as well as the metals of thealkaline earths. W e frequently employ solutions containing several 5 ofthese metals, for example, barium, strontium, and calcium, etc., orbarium, caseium, potassium, etc.; or we may apply alternate coatingsderived from different solutions; for example, an alkaline-earth oxid oroxide in alternation with oxid or oxide of alkali metals.

With regard to the object or substance which is to receive a metallic oroxid coating, we may employ a metal in the form of a wire, filament,ribbon, small tube or rod, plate, or

the like. We have found that nickel, or

nickel-chromium alloys, are good examples of metals which actsatisfactorily in receiving and retaining the coating. Even morerefractory metals, such as tungsten, molybdenum, tantalum, platinum,etc., are also suitable. Our invention, however, is not confined tocoating metals or metallic substances. In some cases we wish to apply ametal or oxid coating to a non-metallic substance such as porcelain,silica, and so forth. There are certaintypes of cathodes (used in thesocalled heater tubes) in which the emitting surface is not identicalwith the heating element, but comprises a tube or small cylinder, etc.,which is heated by means of a separate filament or heater arrangedwithin it. Our

invention is equally applicable to coating such emitting elementsintended to beseparately heated, and may be applied to them whether theyare of metallic or non-metallic composition.

It will be clear from the foregoing description that our process issusceptible of many variations and modifications, all of which comewithin the spirit of our invention.

We claim:

1. Process of coating substances with at least one of the metals fromthe group consisting'of the alkali metals and the alkaline earth metalswhich comprises treating the metal with substantially anhydrous liquidammonia, applying the resulting metal solution to the substance to becoated, and driving off the ammonia to leave a coating con taining themetal.

2. Process of coating substances with an electron-emitting oxid of ametal from the group consisting of the alkali metals and thealkaline-earth metals, which comprises trea ing the metal withsubstantially anhydrous liquid ammonia, applying the resulting metalsolution to the substance to be coated, and converting the resultingcoating to oxid.

3. Process of coating substances with an elect-ron -emitting oxidmaterial derived from a plurality of metals from the group consisting ofthe alkaline metals and the alkalineearth metals, which comprisestreating the metals with substantially anhydrous liquid ammonia,applying the resulting metal solution to the substance to be coated, andconverting the resulting coating to oxid.

4.. Process of coating substances with a plurality of electron-emittingoxides of metals from the group consisting of the alkali metals and thealkalineearth metals, which comprises forming a plurality of solutionsof different metals in substantially anhydrous liquid ammonia, saidmetals being from the group consisting of the alkali metals and thealkaline-earth metals, successively applying the resulting metalsolutions to the substance to be coated, and converting the resultingcoatings to oxid.

5. Process of coating a metallic element of refractory character withelectron-emitting oxid material to form an electron-emitting cathodeelement, comprising treating at least one of the metals from the groupconsisting of the alkali metals and the alkaline-earth metals withsubstantially anhydrous liquid ammonia to form a solution, applying theresulting metal solution to. the element to be coated, and convertingthe resulting coating to oxid.

6. Process of coating a metallic element of refractory character withelectron-emitting oxid material to form an electron-emitting cathodeelement, comprising treating at least one of the metals from the groupconsisting of the alkali metals and the alkaline-earth metals withsubstantially anhydrous liquid ammonia to-form a solution, applying theresulting metal solution to the element to be coated. and converting theresulting coating to o'xid by driving oi the ammonia and heating thecoated element in an oxidizing atmosphere.

7., Process of coating a substance with barium oxid, which comprisestreating barium metal with substantially anhydrous liquid ammonia,applying the resulting barium solution to the substance to be coated,and converting the resulting coating to barium oxid. 8. The. method ofproducing objects having an electron-emitting coating which comprisestreating readily oxidizable metal with substantially anhydrousammoniacal liquid solvent so as to form a composition more fluid thanthe metal, applying the composition to the object so as to form acoating, evaporating the solvent, and converting the metal to oxide.

9. The method of producing objects having an electron-emitting coatingwhich comprises wetting the object with a solution of a readilyoxidizable metal in a substantially anhydrous ammoniacal liquid,evaporating the solvent, and converting the metal tooxide.

10. The method of producing objects having an electron-emitting coatingcontaining an oxide of an alkaline earth metal which comprisestreatingthe metal with substantially anhydrous ammoniacal liquid so asto form a composition more fluid than the metal, applying thecomposition to the object so as to form a coating, and converting themetal to oxide. I

11. The method of producing objects having an electron-emitting coatingcontaining an oxide of a readily oxidizable metal which comprisestreating the readily oxidizable metal with substantially anhydrousliquid ammonia so as to form a composition more fluid than the metal,applying the composition to the object so as to form a coating,

evaporating the solvent, and converting the metal to oxide.

12. The method of producing objects having a. coating containing anoxide of an alkaline-earth metal which comprises treating the metal withsubstantially anhydrous liquid ammonia so as to form a composition morefluid than the metal, applying the composition to the object so as toform a coating, evaporating the ammonia, and converting the metal tooxide.

13. The method of producing objects having an electron-emitting coatingcontaining an oxide of barium, which comprises treating barium withsubstantially anhydrous liquid ammonia so as to form a composition morefluid than barium, applying the composition to the object so as to forma coating, evaporating the ammonia, and converting the barium to oxide.

In testimony whereof, we afiix our signature.

HUGH S. COOPER. MENAHEM MERLUB-SOBEL.

